This verb has the aorist without any connecting vowel so we getπρο+βα+ντ+ς (prefix+stem+suffix+declension) > προβας (dentals "ντ" drop out before the "ς" according to the 3rd declension)I'm not sure but I think the whole list of the aorist participle is:

It almost looks like it's acting like an athematic verb??? (i.e. something similar to a "ui" verb where it doesn't take a connecting vowel.)

I thought the verb was 2nd aorist active (exactly like ἀναβαίνω is 2nd aorist active since the 1s is ἀνέβην) so the nom-sg-masc ending would be "-" (not ς as would be the case if it were 1st aorist active????)

GlennDean wrote:It almost looks like it's acting like an athematic verb??? (i.e. something similar to a "ui" verb where it doesn't take a connecting vowel.)

I thought the verb was 2nd aorist active (exactly like ἀναβαίνω is 2nd aorist active since the 1s is ἀνέβην) so the nom-sg-masc ending would be "-" (not ς as would be the case if it were 1st aorist active????)

Well, it depends on what you mean by "second aorist." If it's just non-first aorists (i.e., sigmatic), then, yeah, βαίνω has a second aorist. But unlike the more common thematic second aorist, the aorist of βαίνω is athematic. So some grammarians call this athematic second aorist a "third aorist," but despite its name it is one of the most primitive verbal forms in the entire Greek verbal system.

David Lim wrote:But I have no idea why the aorist stem fluctuates between "βη" (like in the indicative) and "βα". Can anyone explain this? All I could find so far was Funk 4610.2 and Smyth 488.

The root of this verb alternates between a long-alpha form in βα- which in Attic-Ionic becomes βη- and a short-alpha form in βα- that doesn't change. It is the short-alpha forms of the root that function as the verb stem in the athematic (2nd) aorist participle. The same thing happens with the verb ἵσταμαι, which alternates between a long-alpha form στα- which becomes στη- in Attic-Ionic dialects but remains στα- in the short vowel forms.

GlennDean wrote:It almost looks like it's acting like an athematic verb??? (i.e. something similar to a "ui" verb where it doesn't take a connecting vowel.)

I thought the verb was 2nd aorist active (exactly like ἀναβαίνω is 2nd aorist active since the 1s is ἀνέβην) so the nom-sg-masc ending would be "-" (not ς as would be the case if it were 1st aorist active????)

Yes, it is 2nd aorist active (intransitive) of the athematic type. You won't see the 1st aorist active of βαίνω in the GNT, but it appears in Homer (in the sense "cause to stride"): ἔβησα; the nom. sg. ptc. is βησας, βησασα, βησαν (from βησα-ντ-ς κτλ.).

I'm not sure where you're getting this zero ending from. The masculine sing. nom. 3d decl. ending is -ς, though the neuter sing. nom. 3d decl. is -. The chart posted by David Lim has all the relevant forms.

GlennDean wrote:(or, because it doesn't use a connecting vowel, the ending is ς ? )

Actually it is not like that. For that particular type of third declension which the active aorist masculine participle always uses, the "original" endings are:SN: ς (when the stem ends in ν / ρ / σ / οντ: [τ]ς is removed and last vowel is lengthened)SA: α (when the stem ends with ι / υ / { ι + τ / δ / θ }: same as nominative but with ς replaced by ν)SD: ιSG: οςPN: εςPA: αςPD: σι[ν]PG: ωνThis is why in the active masculine participle the connecting vowel "ο" if present is lengthened to "ω" and "τς" is dropped. Likewise, for the participles with stem ending in "σα", "α" is a long vowel and the "ς" is not dropped, but "ντ" is dropped.Not unexpectedly, "αναβαινω" has aorist participles exactly akin to those of "προβαινω".See Funk 1540.1 and all the other sections for details on the third declension.